1. Field of the Invention
This invention relates to a circuit for converting varying line voltages over a wide range and for selectively supplying either a constant voltage to power a DC motor or a constant current to charge a storage battery.
With portable electric and electronic devices, it is desirable that they can be operated at various voltages, or, if they have storage batteries, that these storage batteries can be charged at various voltages. For example, electric shavers, electronic flash units, portable radios, or the like are often taken along on journeys abroad and are then operated in the various countries with different line voltages. These line voltages generally vary on the one hand between 110 volts and 240 volts and on the other hand between 50 Hz and 60 Hz.
To adapt the small units, and specifically storage batteries, to the various voltages, one needs to transform the voltage, which can be done capacitively or inductively.
2. Description of the Prior Art
A transistor converter circuit is already known, which can generate on the one hand a charging current for a storage battery and on the other hand a higher direct current for driving a motor (DE-OS No. 20 14 377, U.S. Pat. No. 3,568,038). This device has a high-frequency through-flow transformer with a saturable core, whose primary side is connected to the rectified line voltage and whose secondary side supplies the desired current. The known circuit can be operated only at a particular line voltage, and therefore does not automatically adapt itself to different voltages. Since the core of the transformer always reaches the saturation region, its efficiency is low, and there are thermal problems.
Furthermore, a circuit arrangement is known for the controlled supply to a load from input voltage sources of various voltages. This arrangement uses a blocking oscillator type converter, whose primary coil is in series with a switching transistor and an emitter resistor (U.S. Pat. No. 4,005,351). A secondary coil here feeds the load, and feedback is effected through another coil. Another transistor is connected to the base of the switching transistor. The voltage drop at the emitter resistor is applied through a diode to the base of the second transistor. The switch-on time of the switching transistor here depends strongly on the input voltage, i.e. the oscillation frequency of the blocking oscillator type converter depends very strongly on the input voltage and becomes higher with increasing input voltage. To compensate this undesirable circumstance, a relatively expensive control circuit has been provided.
Another known circuit arrangement accounts for the influence of the input voltages on the primary side directly, that is not through the detour via another control circuit and a time delay. This is done by adding another current component to the primary current that is flowing over the emitter resistor. Said current component corresponds to the input voltage, for example, is directy proportional to it (No. P 29 49 421.1-32). As the input voltage rises, the primary current is then shut off earlier, i.e. at a lower value, in such a fashion that the output power has a predetermined dependency on the input voltage, and in particular is dependent on it. A disadvantage of this circuit arrangement is that stabilization of the charge current is not very good.
Finally, another circuit arrangement is known, which uses the voltage drop across the emitter resistor, in order to cut off when a particular primary current is reached (DE-OS No. 27 51 578). In addition, a control voltage is derived from another coil during the blocking phase of a converter. Said control voltage also influences the cut-off time, so that a certain characteristic curve is achieved. As the input voltage increases, the feedback becomes stronger, which counteracts the cut-off through the primary current. The subsequent control circuit therefore must additionally also compensate the larger feedback current.